1. Field of the Invention
This invention relates to the synthesis of hydrocarbons from a methane source. A particular application of this invention is a method for converting natural gas to more readily transportable material using a molten salt methane conversion system which contains a reducible metal oxide and using a feed gas which contains molecular oxygen.
2. Description of the Pertinent Art
A major source of methane is natural gas. Other sources of methane have been considered for fuel supply (i.e., the methane present in coal deposits or formed during mining operations). Relatively small amounts of methane are also produced in various petroleum processes.
The composition of natural gas at the wellhead varies, but the major hydrocarbon present is methane. For example, the methane content of natural gas may vary within the range of about 40 to about 95 volume percent. Other constituents of natural gas include ethane, propane, butane, pentane (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium, and nitrogen.
Natural gas is classified as dry or wet, depending upon the amount of condensable hydrocarbons contained in it. Condensable hydrocarbons generally comprise C.sub.3 + hydrocarbons, although some ethane may be included. Gas conditioning is required to alter the composition of wellhead gas; processing facilities usually being located in or near the production fields. Conventional processing of wellhead natural gas yields processed natural gas containing at least a major amount of methane.
Large-scale use of natural gas often requires a sophisticated and extensive pipeline system. Liquefaction has also been employed as a transportation means, but processes for liquefying, transporting and revaporizing natural gas are complex, energy intensive and require extensive safety precautions. Transport of natural gas has been a continuing problem in the exploitation of natural gas resources. It would be extremely valuable to be able to convert methane e.g., natural gas) to more readily transportable products. Moreover, direct conversion of olefins such as ethylene or propylene would be extremely valuable to the chemical industry.
It has been discovered that methane can be converted to higher hydrocarbons by contacting a methane-containing feed with a body of hot molten salt containing a reducible metal oxide. See commonly assigned co-pending U.S. patent application Ser. No. 747,548, filed June 21, 1985.
In procedures such as those described in said copending application, methane is converted to hydrocarbon products and coproduct water and the reducible metal oxide which is contained in the molten bath is depleted in active oxygen, resulting in a reduced metal oxide which is relatively inactive for the oxidative conversion of methane. It is taught that the reduced metal oxide can be regenerated by separate contact with an oxygen containing gas whereby the metal oxide in reduced form is reoxidized to active form. Such regenerations, however, require separate reaction zones or pulsed or phased reaction conditions which introduce added costs and inefficiencies to the processing sequence.
Mixed feed gases comprised of both methane and molecular oxygen have been used for the conversion of methane to higher hydrocarbons wherein the mixed gases are contacted with beds of contact solid comprised of a reducible metal oxide. See U.S. Pat. No. 4,523,049. The use of fluidized, fixed or moving beds of solids is taught. In such systems there are significant problems of heat transfer, attrition, and the like which have an adverse effect on practicality of the system.